I have been making floats, for several years, following the Chuck Cunninham design outlines.
I am now looking for 'rule of thumb' design parameters for tail dragger floats.

Here's some pictures of a tail dragger float plane 9I had. It's an old Stuntwagon. It's pretty light and has big controls.

I took some regular flam floats and cut them off at the step. The step (end of short float) is positioned in the same position as normal about a half inch behind the CG. I used the left over part of one float to make the tail float. It was attached to the rudder with 4 popsicle sticks and some 2-56 bolts. I did add a little fin on the bottom to help steering. Don't know if I really needed it or not.

As soon as I hit power, the tail picked up and the plane was skimming along. The project was sort of non-eventful. It works good and I didn't notice and problem areas at all. It looks sort of cool, that's about all.

The last 2 pictures of the Magic were where I got the idea. I found them on the web somewhere several years back.

Thanks Ed for your posting. Your half a float solution appears to be in line with my best guess.
Front floats about half length normal floats - IE 37 - 40 % length
Front floats rear end to be at step location - IE 1/2" behind CG
Front floats tip to be about 4" ahead of prop

Rear float - just big enough to prevent the planes rear end from dragging in the water.
As these are for a WW1 Bipe I'll make them flat sided with a bit of a sweep front and rear. Similar to those on the Magic in your pics.
I'm still interested to see if anyone else has an input.

I Googled Images for "Sopwith on Floats"
There is a neat website at www.sopwith-aeroplane.com/CAD_Page.html
with all kinds of detail about float construction. Enough info for scale documentation.